The present invention relates to a printer capable of stably conveying a stencil thermally perforable stencil to master clamping means mounted on an ink drum.
In a stencil printer, a stencil perforated or cut in accordance with image data, i.e., a master is wrapped around an ink drum that is rotated by a drive source. Ink is transferred from the ink drum to a paper or similar recording medium via the perforations of the master, forming an image on the paper. Master clamping means is openably mounted on the outer periphery of the ink drum for clamping the leading edge of the master. At a preselected clamping position, the master clamping means is caused to open and then close to clamp the leading edge of the master conveyed by a platen roller and rollers positioned upstream of the clamping means in a direction of stencil transport.
The stencil may have a laminate structure consisting of a thermoplastic resin film as thin as 2 .mu.m to 9 .mu.m and a porous support formed of Japanese paper, synthetic fibers, or a combination thereof. The porous support may be reduced in thickness or may even be omitted. Conveyance of such a stencil toward the master clamping means has some problems left unsolved, as follows.
(1) Because the stencil is so thin, it is apt to wrap around the platen roller or any other roller being driven before reaching the master clamping means due to, e.g., static electricity or curl. PA1 (2) Even when the stencil is successfully conveyed without wrapping around any roller, it slightly waves due to thermal contraction during perforation or the curl of the film. Should the master clamping means clamp the waved stencil and wrap it around the ink drum, the master might crease on the ink drum and bring about defective printing. This is particularly true when the porous support is thin or when the thermoplastic resin film is used alone, i.e., when the stencil lacks in elasticity. PA1 (3) A guide plate may be provided on a stencil transport path extending to the master clamping means. However, the guide plate must not adjoin the ink drum that is rotated or interfere with the ink drum or the master clamping means. Even the guide plate cannot avoid the defective conveyance of the stencil if the stencil lacks in elasticity and is electrostatically charged. PA1 (4) It is a common practice to coat the surfaces of the stencil with an antistatic agent in order to protect the stencil from static electricity. A greater amount of antistatic agent is necessary for a stencil lacking in elasticity than for a stencil having elasticity. This brings about the corrosion of the thermal head and increases the cost of the stencil itself.
Although technologies capable of solving the above problems (1)-(4) have not been reported yet, Japanese Patent Laid-Open Publication Nos. 59-104937 and 6-320853, for example, each teaches master nipping means capable of nipping the leading edge of a master at a position downstream of cutting means and conveying it to master clamping means mounted on an ink drum. Japanese Patent Laid-Open Publication No. 6-305232 proposes an arrangement of the type conveying the leading edge of a stencil from a position upstream of cutting means to master clamping means while laying the stencil on a sheet member.
However, the master nipping means taught in the above Laid-Open Publication Nos. 59-104937 and 6-320853 each is positioned downstream of the cutting means. It follows that when the leading edge of the stencil from which the master has been cut off is conveyed to the master nipping means, the stencil is apt to wrap around a roller, jam the cutting means, or be caught by the inlet of the master nipping means. This prevents the leading edge of the stencil from being stably conveyed to master clamping means mounted on an ink drum.
The problem with the arrangement disclosed in Laid-Open Publication No. 6-305232 is that the stencil being conveyed together with the sheet member is apt to roll up away from the sheet member and crease before reaching master clamping means mounted on an ink drum. In addition, it is difficult to stably convey the stencil. Although the leading edge of the stencil may be adhered or otherwise connected to the sheet member in order to prevent it from rolling up, this kind of approach needs an extra mechanism and an extra step. Moreover, the stencil is likely to crease during adhesion or similar connection or fail to have its leading edge smoothly peeled off the sheet member when handed over to the master clamping means.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 6-320852.